The most important element of using keywords is not for Hub optima hub performance but to avoid getting the written work labeled as spam.
There are guidelines that the Google Search Engine uses and if an article is loaded with keywords in an effort to attract internet traffic, Google ranks the article accordingly.
From my experience, the articles should have no more than 5 per cent of keywords used in the content. The topic of the article should be between about 70 characters because the Google search engine crawlers respond to less characters. Use the Google Keyword Search Tool to find keywords and the Google Insight for Search to determine if the topic will attract internet traffic. Optimal Hub performance is based on effective keyword usage but also has to be a quality article with an intended online article.